Engineers monitoring BP Plc's damaged well in the Gulf of Mexico detected seepage on the ocean floor that could mean problems with the cap that has stopped oil from gushing into the water, the government's top oil spill official said on Sunday.

Earlier on Sunday, BP officials had expressed hope that the test of the cap which began Thursday could continue until a relief well can permanently seal the leak next month. Oil gushed from the deep-sea Macondo well for nearly three months until the new cap was put in place last week.

But late on Sunday, the U.S. government released a letter to BP Chief Managing Director Bob Dudley from retired Coast Guard Admiral Thad Allen that referred to an unspecified type of seepage near the mile-deep (1.6 km-deep) well along with "undetermined anomalies at the well head."

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"I direct you to provide me a written procedure for opening the choke valve as quickly as possible without damaging the well should hydrocarbon seepage near the well head be confirmed," Allen wrote.

BP did not respond to requests for comment on Allen's letter.

The worst oil spill in U.S. history has caused an economic and environmental disaster in five states along the Gulf Coast, hurt President Barack Obama's approval ratings and complicated traditionally close ties with Britain.

Those concerns are sure to be discussed when British Prime Minister David Cameron meets Obama in Washington on Tuesday.

The plan had been for BP to resume siphoning the oil after the completion of the pressure tests on the well, which extends 2.5 miles under the seabed, to judge if it is able to withstand the process to seal the leak.

But Doug Suttles, BP's chief operating officer, said the company now hopes to keep the damaged well shut until the relief well is completed in August and the leak is sealed off with heavy drilling mud and cement.

"We're hopeful that if the encouraging signs continue that we'll be able to continue the integrity test all the way to the point that we get the well killed," he told reporters before Allen issued his statement. "Clearly we don't want to reanimate flow into the Gulf if we don't have to."

Photo shows a pressure gauge in this image captured from a BP live video as
BP launches a critical pressure test on its ruptured Gulf of Mexico oil
well on July 15, 2010.